Last season, Kaepernick, the former San Francisco 49ers quarterback, set off a national debate when he knelt during the national anthem before games, as a way to shine a light on police violence and racial injustice. Now a free agent, he has been unable to find a job, leading to speculation that teams are steering clear of him because of his controversial stance.

If Kaepernick were a basketball player, things might be different. In the N.B.A., the biggest stars routinely share their opinions on a range of thorny social issues. Top coaches, including Steve Kerr of the Golden State Warriors and Gregg Popovich of the San Antonio Spurs, have been critical of President Trump. The league’s commissioner, Adam Silver, was widely lauded for permanently barring Donald Sterling, the former owner of the Los Angeles Clippers, who was caught on tape using racist language.

But Kaepernick is trying to land a job in the N.F.L., a league filled with players who talk proudly about toeing the line and keeping their heads down. Coaches and team owners have an almost allergic reaction to addressing political issues. The commissioner, Roger Goodell, speaks often about patriotism. Few leagues spend as much time wrapping themselves in the flag.

Never mind that N.F.L. teams have offered million-dollar contracts to players convicted of animal abuse, domestic violence and other transgressions. Kaepernick, it would appear, crossed an immutable line, and he is now paying a price as teams hire journeyman backups and younger, untested quarterbacks over someone who just a few years ago led his team to the Super Bowl.

“He’s being singled out and not offered employment because of what he did,” said Richard Lapchick, the director of the Institute for Diversity and Ethics in Sport at the University of Central Florida. “The first time he did it, I thought: This will be his last season.” The reason? “I felt I knew the political reality in the N.F.L.”

The political reality is this: The league consists of a largely conservative group of owners and, according to polls, a majority of fans who did not support Kaepernick’s actions. One poll suggested that some fans watched less football last season because of the anthem protest, which spread to a handful of other players, helping push down television ratings over all. Some fans, like Dave Ippolito, said they stopped watching football entirely last season.

Ippolito, a retiree who lives not far from Cincinnati and is a lifelong Giants fan, said he did not oppose Kaepernick expressing his views off the field. But by protesting on the field, he said, he dragged politics into the game.

“If you want to do stuff in your off hours, that’s fine, that’s your right as an American,” Ippolito said, adding: “It’s time to play football. It’s not time for politics.”

Ippolito’s views are far from universal. Some fans, in fact, applaud Kaepernick. Last month, he had the 17th best-selling jersey in the N.F.L.

Those who believe Kaepernick is being shunned point to the fact that about 20 other quarterbacks have been signed so far this off-season, including Mike Glennon, who received a three-year, $45 million contract from the Chicago Bears even though he threw only 11 passes the past two seasons; Josh McCown, who signed a one-year, $6 million contract with the Jets even though he is 37 and played for the one-win Cleveland Browns last season; and Geno Smith, who received a $1.2 million, one-year contract from the Giants after a dismal showing with the Jets.

Some football analysts say Kaepernick’s quarterback rating suggests his skills have declined. His rating slipped to a mediocre 55.2 last season, from a high of 76.9 in 2012, the year before he led the 49ers to the Super Bowl. His backers, though, note that Kaepernick played on dreadful teams in the past two seasons, when the 49ers won only seven games under two different head coaches.

Others point to his style of play, which includes scrambling out of the pocket, something many coaches eschew. Yet Michael Vick, who also scrambled, found work in past years with three teams after he missed two seasons while serving a jail sentence for animal cruelty.

Then there is money. Kaepernick was paid $14.3 million last year, and teams may assume he wants to continue to be paid like a starter.

Though he has gone unsigned for more than three months, Kaepernick, now 29 with six years of experience in the N.F.L., may yet land a job, even once the season begins in the fall, particularly since he appears willing to be a backup, not just a starter. Quarterbacks are injured all the time, and teams may view him differently if they need a replacement in a pinch.

For now, though, Kaepernick is not getting on a team. Only the Seattle Seahawks invited him for an interview. In one of the odder compliments, Coach Pete Carroll said afterward that Kaepernick should be a starter in the N.F.L., but the Seahawks already had a starter, so they were going to pass. Kaepernick’s representatives say he has never insisted on being a starter.

At least one of Carroll’s players, Michael Bennett, was skeptical of his being passed over, saying Kaepernick’s inability to get a new contract is evidence of the racial divide in the N.F.L., a league in which about three-quarters of the players are black.

If Kaepernick does not find another job in the N.F.L., he will join the list of athletes ostracized for political outspokenness.

One of them, Craig Hodges, a sharpshooting player on the Chicago Bulls in the early 1990s, never played in the N.B.A. after he spoke openly about police brutality and gave the elder President George Bush a handwritten letter when his championship team visited the White House.

“I couldn’t play anymore at 32, and I was a three-time 3-point-shooting champion and a two-time world champion,” said Hodges, 56, who is now a basketball coach at the high school he attended in Chicago. “It was a point in my life where I took a stand for a lot more people from where I was from.”

Hodges said things were different now, thanks to social media, which has made it easier for players to get out their side of a story. (Also, the N.B.A. has evolved into one of the more politically progressive leagues.)

Kaepernick has declined requests for interviews, so his thoughts on why he has not been hired must wait for another day.

But on Twitter, he continues to share his thoughts, including on Friday after the officer accused of killing Philando Castile in Minnesota was acquitted.

Kaepernick called for the dismantling of “a system that perpetually condones the killing of people, without consequence.”

Kaepernick has also tried to show he is not alone. On Instagram he posted a picture of several boxes of letters he described as fan mail.

A version of this article appears in print on , on Page D1 of the New York edition with the headline: Drawing a Hard Line on Political Views. Order Reprints | Today’s Paper | Subscribe